Rapid progress has been made in terms of achieving paperless systems in offices. Paper documents created on a personal computer and old paper documents that have been accumulated in a binder or the like may now be stored in a database by converting these documents to electronic documents such as image data by means of a scanner.
Materials distributed at a meeting even now are preferred to be paper documents, and there are also many opportunities for electronic documents that have been stored in a database to be printed out as paper documents which are then delivered to users.
Consider a case where a user who has received a paper document wishes to archive or transmit the document electronically or wishes to extract content from the document and reutilize it. If instead of using data obtained by putting the paper document back into electronic form it were possible to acquire the original data from a database and utilize this data, then convenience would be enhanced to the extent that loss of information through intermediate use of paper documents is eliminated.
A system that has been proposed in order to meet this need reads a paper document by a scanner and retrieves data that is similar in content from a database, as described in the specification of Japanese Patent No. 3017851.
In order to raise retrieval accuracy, the applicant has given consideration to dividing a read document (an input image) into text information and non-text information (photographs and pictures, etc.) using an area identification technique (which may be well known, for example, the technique described in the specification of U.S. Pat. No. 5,680,478) and using similarity-degree calculation processing that conforms to the characteristic of the particular information.
More specifically, the approach includes extracting a text area and a photograph area from a page image of a read document and a page image of a registered document by area identification processing, obtaining degree of similarity, which is obtained based upon the feature of a character string that has undergone character recognition processing, in regard to the text areas, and obtaining degree of similarity with regard to the photograph areas using image features such as color and edges, etc.
In particular, since a photograph or picture contained in a document page represents the feature of the page to a large extent, it can be anticipated that thus finding the degree of similarity of a photograph or picture in highly precise fashion will contribute to a major improvement in the performance of the retrieval system.
On the other hand, in relation to finding the degree of similarity between an input image and a registered image using an image feature in such a retrieval system, it is essential that the orientations of both images be the same. If the orientations are different, a high retrieval precision cannot be expected.
However, if a paper document has been read using a scanner or the like, there is a possibility that the orientation of the input image acquired will change in any of four directions depending upon the manner in which the paper is placed on the document glass. Forcing the user to place the paper in the correct manner for the purpose of solving this problem may detract from the user friendliness of the system. Moreover, depending upon the paper size, there are cases where scanning in the desired direction may not be possible owing to the structure of the document glass or automatic document feeder.
There is a method available for avoiding the problem relating to image orientation. Specifically, when degree of similarity of an image feature is calculated, a difference in image orientation is taken into account and four types of images obtained by rotation through angles of 0°, 90°, 180° and 270° with respect to the image on the input side are prepared, or four types of features are created by performing a feature coefficient conversion. Degree of similarity is then found and whichever image or feature has the highest degree of similarity is employed. Regardless of the method used, however, processing for obtaining the degree of similarity must always be executed four times and the processing time required increases fourfold.